A device for balancing positive and negative ion output in bipolar generators is known which comprises a bias electrode (e.g. grid) and a controlled bias voltage source, the high voltage terminal of the such generators being connected to the bias electrode, while the low voltage terminal thereof is grounded (refer to US patent Application Publication 2006/0072279 A1).
Also a device is known for automatic balance control of positive and negative ions at the output of bipolar ion generator (refer to U.S. Pat. No. 4,618,249). The device includes a conducting grid connected to two variable resistors via diodes with different polarity connection. The resistance of the resistors is adjusted by a signal applied from a special balance sensor (electrometer) so that balance between positive and negative ions at the object is achieved.
The drawbacks of this technique are as follows:
1. Low sensitivity of the ion balance sensor which is more distanced from the ionizing electrode than the bias electrode (grid).
2. Low accuracy of ion balance adjustment due to the stepped adjustment of the resistor resistance values.
3. Complexity of realization of the device.
Further, a device is known for automatic ion balance adjustment in bipolar ion generators (refer to U.S. Pat. No. 6,850,403 B1). The device includes: bias electrode (grid), regulated bias voltage source isolated from ground; the high voltage terminal of the said voltage source being connected to the bias electrode, while the low voltage terminal thereof is grounded via a balance sensor. The balance sensor is a resistor. The sensor output voltage which is proportional to the bias current is compared with the reference voltage to generate a correction signal which is applied to the control input of the bias voltage source.
The drawback of the technique is the low sensitivity of the ion balance sensor caused by the bias current and the earth leakage currents from the bias voltage source which is isolated from the ground flowing through the sensor.
The leakage currents from the isolated voltage source are much higher than the bias current; moreover they are strongly dependent on ambient temperature and humidity.